The need for this invention arose from problems in serially communicating information between radio subsystems in an internally-bussed two-way radio. Specifically, the problems to be addressed were those of transmission economy, bus contention, error detection and subsystem device addressing.
Register modeling and serial linkage of radio subsystems is at the heart of the architecture for the latest generation of two-way radios. The radios are constructed by interconnecting independent radio subsystems, control heads and accessories with the basic R.F. modem over a bidirectional serial link, as illustrated in FIG. 1.
The serial link interfaces with the radio devices via standard Universal Asynchronous Receiver/Transmitter (UART) devices. Thus, message protocols must be based upon sequences of eight-bit words, each with start and stop bits, as is customary with these devices.
Others have attempted to solve the problem by asynchronous ASCII character packet transmission, randomized bus access retry, checksum or cyclical redundancy check error detection, and source and destination addressing.
A general purpose protocol has the ability to communicate data and control information between devices. Typically, a general purpose protocol defines a primitive set of operations that a device can perform. An operation code is usually transmitted near the beginning of a message to instruct the recipient as to how to treat the remaining transmission.
The currently accepted methodology is to reserve a unique eight-bit word within the permissible character set as a recognizable start-of-message. Its meaning is reserved and will, therefore, not appear anywhere else in the message. The start-of-message contains no other information and owes its sole existence to designating the legitimate start of the message. For example, a message that is start and aborted due to collision or device failure, can confidently be ignored when the next start-of-message word is seen. This is infinitely preferable to the reception and misinterpretation of an aborted message. Accordingly, the start-of-message word must maintain its uniqueness by not serving any other function--it must be a reserved word.
A multi-device, multi-access protocol must also reserve a number of words to identify various devices on the serial bus. To provide for acknowledgment of received messages and handshaking between devices, often, protocols transmit both the address of the originating device and the address of the device to which the message is destined.
For error detection, many protocols append a checksum or other error detection contrivance.
In multiple access busses, collision avoidance and bus contention must be anticipated and orchestrated. Many systems provide for randomized retry for bus access, hoping that randomization will reduce the possibility of further collisions.
However, each of these considerations take their toll in protocol design sacrificing transmission efficiency with starts-of-messages, source addresses, destination addresses, and checksums and requiring bus contention randomization overhead.
The instant invention solves these problems by economizing and optimizing transmissions, eliminating randomization overhead, simplifying bus contention and simplifying device addressing.
This invention represents a significant advance over the prior art and over this technical field by providing an economized, optimized serial communications protocol with simplified addressing and bus contention.